This invention relates to a controller for a linear motor used for driving a compressor and in particular but not solely a refrigerator compressor.
Linear compressor motors operate on a moving coil or moving magnet basis and when connected to a piston, as in a compressor, require close control on stroke amplitude since unlike more conventional compressors employing a crank shaft stroke amplitude is not fixed. The application of excess motor power for the conditions of the fluid being compressed may result in the piston colliding with the cylinder head in which it is located.
In International Patent Publication no. WO01/79671 the applicant has disclosed a control system for free piston compressor which limits motor power as a function of property of the refrigerant entering the compressor. However in some free piston refrigeration systems it may be useful to detect an actual piston collision and then to reduce motor power in response. Such a strategy could be used purely to prevent a compressor damage, when excess motor power occurred for any reason or, could be used as a way of ensuring high volumetric efficiency. Specifically in relation to the latter, a compressor could be driven with power set to just less than to cause piston collisions, to ensure the piston operated with minimum head clearance volume. Minimising head clearance volume leads to increased volumetric efficiency.
It is an object of the present invention to provide a linear motor controller which goes someway to achieving the above mentioned desiderata.
Accordingly in one aspect the invention may broadly be said to consist in a free piston gas compressor comprising:
a cylinder,
a piston,
said piston reciprocable within said cylinder,
a reciprocating linear electric motor derivably coupled to said piston having at least one excitation winding,
means for obtaining a measure of the reciprocation time of said piston,
means for detecting any change in said reciprocation time, and
means for adjusting the power input to said excitation winding in response to any detected change in reciprocation time.
Preferably said motor is an electronically commutated permanent magnet DC motor.
Preferably said compressor further comprises back EMF detection means for sampling the back EMF induced in said at least one excitation winding when exciting current is not flowing, and zero crossing means connected to the output of said back EMF detection means and means for determining the time interval between output pulses from said zero crossing detection means to thereby determine the time of each half cycle of said piston.
Preferably two successive half cycles of said piston operation are summed to provide said reciprocation time.
Preferably means for detecting any change in said reciprocation time includes means to detect said reciprocation time from a filtered or smoothed value, to provide a difference valve and if said difference value is above a predetermined threshold for a predetermined period, said means for adjusting the power is configured to reduce the power input to said excitation winding.
In a second aspect the present invention may broadly be said to consist in a method of preventing overshoot of the reciprocating portion of a linear motor comprising the steps:
determining the reciprocation time of said reciprocating portion,
detecting any change in said reciprocation time, and
adjusting the power input to said linear motor in response to any detected reduction in reciprocation time
Preferably said reciprocating portion comprises the armature of said linear motor.
Preferably said step of determining said reciprocation time includes the step of detecting zero crossings of the current in said linear motor and determining said reciprocation time from the time interval there between.
Preferably said step of detecting any change in said reciprocation time includes the step of deducting said reciprocation time from a filtered or smoothed value, to provide a difference valve and if said difference value is above a predetermined threshold for a predetermined period, reducing the power input to said linear motor.
In a third aspect the present invention may broadly be said to consist in a controller for a linear motor including an reciprocating portion, said controller adapted to implement at least the following steps:
determining the reciprocation time of a reciprocating portion,
detecting any change in said reciprocation time, and
adjusting the power input to said linear motor in response to any detected reduction in reciprocation time.
Preferably a reciprocating portion comprises the armature of a linear motor.
Preferably said step of determining said reciprocation time includes the step of detecting zero crossings of the current in a linear motor and determining said reciprocation time from the time interval there between.
Preferably said step of detecting any change in said reciprocation time includes the step of deducting said reciprocation time from a filtered or smoothed value, to provide a difference valve and if said difference value is above a predetermined threshold for a predetermined period, reducing the power input to said linear motor.
To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.
The invention consists in the foregoing and also envisages constructions of which the following gives examples.